The present invention relates to a humidity sensor for measuring the moisture content of an atmosphere by means of change in an electrical resistance of a moisture-sensitive layer. The humidity sensor can be used, for example, for measuring the moisture content of air; the moisture content of exhaust gas in an exhaust pipe of an internal combustion engine of, for example, an automobile, ship, or airplane; or the moisture content of, for example, an atmosphere surrounding a fuel electrode or an air electrode or an atmosphere in a pipe of a fuel cell.
Typically employed humidity sensors include resistance-variable-type humidity sensors utilizing adsorption and desorption of water molecules; and capacitance-variable-type humidity sensors utilizing change in capacitance. Such humidity sensors are formed from a moisture sensitive material such as Al2O3, MgCr2O4xe2x80x94TiO2, TiO2xe2x80x94V2O5, or ZrCr2O4xe2x80x94LiZrVO4. Most of the humidity sensors measure a change in electric resistance.
Humidity sensors formed from such moisture sensitive ceramic materials are typically fabricated by a method described in, for example, Japanese Patent Publication (kokoku) No. 1989-22965. Specifically, a lower electrode is formed on an insulating substrate; a moisture-sensitive layer is formed through pelleting or a like process in such a manner as to overlie the lower electrode; and an upper electrode is formed on the upper surface of the moisture-sensitive layer. A humidity sensor whose detection electrodes each assume a comblike shape is also known.
Such conventional humidity sensors can be used in the ordinary atmosphere or the like without involvement of problems. However, the conventional humidity sensors involve problems, such as low-precision measurement and impaired response, in application to measurement in an atmosphere which exhibits significant temperature variation (e.g. xe2x88x9240 to 750xc2x0 C.), very low oxygen concentration, and significant variations in, for example, gas flow velocity, gas flow rate, and gas pressure, as in an exhaust pipe of an automobile, or in application to measurement in an atmosphere which contains a considerable amount of reducing gas, such as an atmosphere surrounding a fuel electrode of a fuel cell. In application to measurement in such an atmosphere, a humidity sensor must exhibit excellent thermal shock resistance in addition to sufficiently good response. A conventional humidity sensor including a moisture-sensitive layer formed through pelleting or a common thick-film-type humidity sensor fails to detect accurately and at highly good response a change in electrical resistance of the moisture-sensitive layer which is induced by the moisture content of an atmosphere to be measured, and fails to exhibit sufficient strength in relation to thermal shock resistance, etc.
The present invention contemplates solving the aforementioned conventional problems. An object of the present invention is to provide a humidity sensor capable of exhibiting highly good response and excellent strength in relation to, for example, thermal shock resistance, and capable of maintaining stable moisture sensitivity over a long period of time, even in application to measurement in an atmosphere which exhibits significant temperature variation, very low oxygen concentration, and significant variations in, for example, gas flow velocity, gas flow rate, and gas pressure, as in an exhaust pipe of a vehicle.
When the moisture-sensitive layer of a humidity sensor assumes the form of a thin layer having a certain thickness, the humidity sensor can exhibit enhanced response, and thermal stress can be reduced. When the detection electrodes of the humidity sensor are formed of a porous material that is formed from a noble metal, such as Pt or Au which exhibits excellent heat resistance and corrosion resistance and has an average pore size greater than that of the moisture-sensitive layer, strong adhesion can be attained among an insulating substrate, the detection electrodes, and the moisture-sensitive layer while excellent response, for example, is maintained intact. Even in application to measurement in a specific atmosphere as in an exhaust pipe of a vehicle, such a humidity sensor can exhibit high accuracy, highly good response, and sufficient strength in relation to, for example, thermal shock resistance, and can maintain excellent humidity detection performance over a long period of time.
The present invention has been accomplished on the basis of this finding.